An LNG facility employing an enhanced fuel gas control system. The enhanced fuel gas control system is operable to produce fuel gas having a substantially constant Modified Wobbe Index (MWI) during start-up and steady-state operation of the LNG facility by processing one or more intermediate process streams in a fuel gas separator. In one embodiment, the fuel gas separator employs a hydrocarbon-separating membrane, which can remove heavy hydrocarbons and/or concentrate nitrogen from the incoming process streams.