NOVELTY - Feed gas stream comprising methane and nitrogen, is cooled in heat exchanger (4). The cooled gas stream is rectified in double rectification column (12) having high and low pressure rectification columns (14,16). A portion of gas stream is expanded into column (14). The operating pressure of column (16) is periodically increased with increase in mole fraction of nitrogen. Thus gas stream is rejected.    USE - For rejecting nitrogen from feed gas stream comprising methane and nitrogen to form methane product.    ADVANTAGE - The vapor loadings of the high and low pressure rectification columns are reduced effectively during the operating life time. Increasing the operating pressure of the low pressure rectification column provides a concomitant increase in the operating pressure of the high pressure rectification column. Increasing the operating pressure of the high pressure rectification column enables more vapor per unit time to be received for operating at a given constant percentage of flood and also reduces the proportion of the feed gas stream which passes out of the expansion device in the liquid state. Therefore the vapor loadings of the high pressure rectification column are reduced effectively. Since the temperature-enthalpy profile of the stream cooled in the heat exchanger closely matches with the temperature-enthalpy profile of the stream warmed in the heat exchanger, the heat exchanger has good thermodynamic efficiency. The reduced vapor loading of the rectification column provides high recovery of methane product. The high reflux of the double rectification column makes easier to separate the methane product. The amount of nitrogen recycled from the low pressure rectification column is reduced to minimize the power consumption of the compressor. The nitrogen is recycled effectively from low to high pressure rectification columns. The apparatus for rejecting nitrogen from feed gas stream has desirable size and reduced power consumption.    DETAILED DESCRIPTION - A feed gas stream comprising methane and nitrogen, is cooled in a main heat exchanger. The cooled gas stream is rectified in a double rectification column which has high and low pressure rectification columns, and a condenser-reboiler (18) which places the high pressure rectification column in heat exchange relationship with the low pressure rectification column. A portion of the feed gas stream is expanded into the high pressure rectification column and the feed gas stream is partially liquefied in the upstream of the double rectification column. The operating pressure of the low pressure rectification column is periodically increased in response to increase in the mole fraction of nitrogen. Thus the feed gas stream is rejected so as to form a methane product.    An INDEPENDENT CLAIM is also included for an apparatus for rejecting nitrogen from the feed gas stream which has a main heat exchanger for cooling the feed gas stream, double rectification column for rectifying the feed gas stream and an expansion device provided downstream of the main heat exchanger communicating with the high pressure rectification column. The expansion device is arranged to introduce a portion of the feed gas stream into the high pressure rectification column in liquid state. A back pressure regulating valve (58) associated with the low pressure rectification column is operable to increase the pressure in the low pressure rectification column.    DESCRIPTION OF DRAWING(S) - The figure shows the schematic flow diagram of the apparatus for injecting nitrogen from feed gas stream.    main heat exchanger (4)    double rectification column (12)    high and low pressure rectification columns (14,16)    condenser-reboiler (18)    back pressure regulating valve (58)