I've removed the LPG kit from my old car.
The valve on the tank will be closed with the ignition off so undoing the fill and fuel pipes from the valve will not result in a woosh of escaping gas. Just the gas that's in the pipes.
If you can't run the engine until the tank is empty, your best bet is to unbolt the valve from the tank until it leaks and allow the gas to vent to the atmosphere. Sounds a bit dangerous, but so long as the gas can vent to the atmosphere and you're no smoking whilst you do it, you'll be safe enough. There will be some frosting up around the area of the leak.
I've had to do it that way before, left it going slowly overnight and it was fine.
Once it's empty just undo the straps and or bolts holding the tank it and lift out.
