Put them in a small chamber and release Freon Gas into the chamber. It is actually a powerful anesthetic and will put you to sleep and as the concentration builds up, the heart will go into a fatal arrhythmia. Game over.
There is a problem with using gas, in that to some extent, a painless death requires the cooperation of the condemned. That is, they are supposed to try and hold their breath, and then inhale a bunch of concentrated cyanide, which kills them very quickly.
However, there was a case not too long ago, where the condemned continued to breathe normally. He started breathing harder, and his skin turned flush, as he very painfully asphyxiated.
A trouble with freon is that it can cause nausea and vomiting as well as seizures. If it was just a case of knocking them out quickly until they asphyxiate, then ordinary carbon dioxide or nitrogen gas would do the trick.