Nissan sentra 2004 overheating, fan and radiator has been changed and also removed is thermostats. What could that be?

I am assuming you flushed out the entire cooling system after you rebuilt the engine.
Did you examine the water pump? If that isn't working correctly it won't be moving the coolant around the engine fast enough to keep it cool.
Have you had the vehicle scanned with an OBD2 scanner? This will find out if it is a sensor that is causing the problem. If you have to pay for this service ask at 4 or 5 different places as prices can be VERY different.

I dont think this is connected to the alternator, unless you have been removing wires or hit the thermostat switch in the radiator or hit the thermostat housing and the valve is blocked up. if its boiling up and the fan is not kicking in and the main pipe from the top is cold then the thermostat is not working, if all the pipes are hot and the fan is not turning on then its the thermo switch on the radiator or the motor has decided to go.

Verify the sending unit is correct. Verify it is overheating. Sometimes the sending unit and/or gauge is bad.

Check your thermostat ,then if that's not it,check your water pump.

Some thing too check:
Make sure coolant level is up to line and clean (not dirty, do to clogged radiator, may need to be flushed or replaced).
Make sure air flow through the out side of radiator is clear (dirt, leaves, ect..) .
Make sure fan is working properly equip t (is not running too slow). Or Manual (belt driven) fan clutch is working properly, not loose or spinning too freely?
Check for broken or cracked fan blades itself.
Check radiator cap, (no gasket tears).
Could be thermostat going bad ?
Could be coolant temperature sensor/switch (from computer) going bad?
Check radiator hose(s) (too soft, may collapse)?
In some rare cases:
Timing could be off.
Could be water pump.
Could be blown head gasket(s).
Could have cracked head(s).
ECT..............Hope this was some help???

have radiator system flushed at a radiator repair shop

let the engine get cold, and remove the radiator cap, fill the system to proper level and re test. run engine until thermostat opens, fans come on, then alow to cool again and re check engine coolant. If this does not work, or the thermostat does not open the thermostat is instaled backwards, remove and reposition.

it sounds like a water pump not circulating antifreeze properly

One of the common things that gets missed at times is the radiator is down in cooling antifreeze. The car will act as normal during the warm up cycle and then the temperature guage will rise and then it will slowly come back down as the thermostat opens and/or the electric cooling fans come on. In actuality the engine is overheating, but it is not boiling over. The level of abtifreeze is at just the right level of lowness that you experience this phenomenon. After the engine cools off , (DO NOT REMOVE WHEN HOT - YOU CAN GET SCALDED WITH BOILING WATER/ANTIFREEZE)), remove the radiator cap and check the level within the radiator and also the antifreeze reservoir level. You can buy a 50-50 mix of antifreeze and water that is ready to put into the radiator. If the level in both the radiator and reservoir are correct, you may have a bad thermostat or some other problem.
Bob - 2003 Nissan Sentra SE-R SPEC V

does the car overheat at idle or travelling down the road? if the thermostat was not changed, thats first step. checking it is replacing it. possibly air-bound. then i would suspect a headgasket problem.