1996 Seville wont start

Btw I changed the battery the belt cleaned the throttle body it isn't the alternator because I still have great power and the starter spins just fine I have the same ignition cylinder but lost the original key and didn't know the right key value to the car. A locksmith made me a key from the vin and I bought a bypass module. Well now I think I found the right key value cause it cranks and at first it didn't. Didn't change the battery cables though and the battery has 12.2 voltage but wont start it just spins the starter

The head gasket is blown , put a fork in it , it's done

Junk it! You have the Northstar engine and they all blow head gaskets! The overheating and white "smoke" , water vapor, from the tailpipe are the tell tale signs! Your looking at upwards of $3,000 to fix it and you have to get someone who is familiar with these engines and vehicles to do the job! For starters the engine and transmission must be removed from the bottom of the vehicle! There's no other way, the heads will NOT come out through the top. These engines were junk when they were new! I'm suprised it's lasted this long! Time to move on to something newer!! Jim