Removing and replacing a propane refrigerator

Congrats on the refrigerator replacement. A conversion to a residential is generally considered an upgrade around these parts. In time you can add an inverter and a couple of batterys and you wont miss the propane unit a bit.

Re the line, find the appropriate sized and threaded cap and cap it off. Problem solved and you'll have a better fridge out of the deal.

Okay, I will ask. WTF is a CNG?

Make sure the CNG is off at the tank, then after installing the plug with pipe sealer or TFE tape turn on the CNG and test for a leak with liquid soap an water in a spray bottle, re-test again after some time has passed.

..

As most units have black pipe gas lines that then change to copper near the device, I would remove the copper line where it enters the steel pipe and replACE it with a plug. Use some pipe dope and check for leaks