Yes it was, but this is different. In all honestly, I don't remember the name of the gas, but what it does is cuts off oxygen in the blood flow. What this does is, while still cycling blood to the brain, it deprives the brain of oxygen, thus giving off a euphoric feeling. The person's last moments are happy and without pain or suffering. In the documentary, this dude went into a chamber where it felt like the altitudes were like they were in space. He got giddy for a while and then kinda just say there chilling. They told him to put on his mask but he didn't give a shit. Another 30 seconds and he would've died. Then, some doctor in India started using a gas that gave off this same feeling to painlessly kill livestock.
I'll go try to find the chunk of the documentary that shows this, just so you don't have to watch the whole thing.
edit: Watch from minute 37 to the end. Only 12 minutes then. BTW, just you get what your watching, when oxygen does not get to the brain, the process is called hypoxia. The only time we used hypoxia in capital punishment is when we used cyanide pellets to kill people, and yes, it was barbaric.