Abstract: Nitrogen is rejected from a pressurised feed gas stream comprising methane and nitrogen by cooling the feed gas stream in a main heat exchanger and rectifying the cooled feed gas stream in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure column in heat exchange relationship with the lower pressure column. A pump withdraws a methane product stream from the column. The methane product stream is pressurised by the pump and is vaporised in the main heat exchanger. The pressurised feed gas stream is expanded with the performance of external work in an expansion turbine 6 upstream of the main heat exchanger.