“Raw” natural gas from the well head often contains some propane, ethane and some less desirable stuff like water, Nitrogen, CO, CO2, H2S, etc.
Natural Gas after the gas processing plant and what is delivered to your home is nearly pure Methane, CH4. Propane is a different molecules, C3H8.
When it gets cold, the vapor pressure changes a lot. When it gets cold enough, the liquid in the pressure tank won't produce enough pressure to feed the fuel system.
