Mate, if you're going to die at some point your heart has to stop beating. If that's through a heart attack, just stopping, who knows, and it could be different for different people.
But the point of any of the peaceful methods is that by the time that happens you're out cold, so you don't know anything about it.
SN knocks you out, then eventually you stop breathing and your heart stops.
N - knowcks you out then depresses your SNS so you stop breathing and your heart eventually stops.
Inert Gas - knocks you out, then eventually your heart stops and you stop breathing.
CO - same
Etc.
Conversely - jump off a building, hit the floor, hope you're out cold before you bleed out or were killed outright.
Train - same.
Hanging - hope you're out fast and itdoesn't go wrong and leave you with brain damage, or you suffer horribly.
Drowning - breathe in water to your lungs, suffer extreme pain and hope you pass out before it's unbearable.
Etc.
It's up to you to choose, but when you die in some way, shape or form your heart stops - if you are out of it and don't suffer, that's theaim for most people.