The best method is to use hypoxia(slowly depriving the brain of oxygen)....pilots feel it on high altitude flights....moutain climbers also face it.....when your brain is slowly deprived of oxygen you will start feeling euphoric....its actually a fun way to die....but to mimic the effects of a high altitude flight on ground level is really difficult and expensive....but I think people are working on a gas(contains some inert gasses) that can solve the problem....But the pro-death penalty group in the US are wacky right-wingers and most of them want the prisoner to suffer the pain of their victim!