I'd probably remove the battery as I'm pretty certain it's not supposed to do that and if it's going to spontaneously combust I'm sure you wouldn't want it to do so in the car.
What do you mean you're 'pretty certain'
Clio_fool is 100% certain that your battery should not smoke.
Given this is nearly 12hrs ago now I assume you either removed the battery or your car has been on fire for a while.
Either your alternator is overcharging the battery (failed regulator) or your alternator diode pack has failed so is now a hard short across your battery - these would be my main guesses other than perhaps the main cable to the alternator/starter might have rubbed on something so is now shorting.
Either way, I hope you got the battery off asap. If it is the first option, so the alternator is overcharging you do not want to drive the car at all. Once the battery goes dry there is nothing to keep the alternator output voltage in check - alternators with failed regulators can putout well over 50V at which point every electronic part in your car will be toast.
If its the second or third option turning the engine off won't stop the magic smoke.