Why does my 2005 impala leak dexcool like crazy, which throws my heat off.

There are several areas that these cars have issues with coolant leaks first off is this car leaving a puddle of coolant on the ground?

If not check the oil and make sure after it's been run awhile that the oil doesn't resemble chocolate milk if it does then the coolant is leaking internally and could cause sever internal engine damage if not addressed promtly. The engine damage occours due to pitting and rusting in the lifters rod/main and cam bearings, oil and water don't mix. Check the oil after the engine has been run a while since if the car sit's the oil and water will separate and the oil may look ok other then being overfull slightly.

Another area that is comon for leaks is the overflow tank if this type has the pressure cap on the tank I have had several bad tanks like this.

The water pump may be leaking out the weep hole located behind the pulley and should be checked with a pressure tester,light and a mirror.

But probably the most comon leak of all is the intake gasket leaking out both one or both ends. Dex-cool adds to the likely hood of these gaskets being the problem since it is citrus based and has had huge classaction lawsuits against GM in the past. Since this car is no longer under warranty the best advice I can give you is to flush out the dex-cool and replace with a good quality 150,000 mile 5 year green like Peak antifreeze or equivalant. But first find the laeak and have it repaired.