NOVELTY - A gas stream (101) containing methane and at least 4 % nitrogen is treated by providing a first membrane unit containing a first membrane having a first feed side and a first permeate side; and providing a second membrane unit containing a second membrane having a second feed side and a second permeate side.    USE - For treating a gas stream containing methane and at least 4 % nitrogen.    ADVANTAGE - The invented method enables the nitrogen content of the gas to be reduced without requiring the membranes to be operated at very low temperatures.    DETAILED DESCRIPTION - Treatment of gas stream comprising methane and at least 4 % nitrogen comprises providing a first membrane unit containing a first membrane having a first feed side and a first permeate side, where the first membrane is more permeable to methane than to nitrogen; providing a second membrane unit containing a second membrane having a second feed side and a second permeate side, where the second membrane is more permeable to methane than to nitrogen and the second membrane unit is connected in series with the first membrane unit such that gas leaving the first feed side can enter the second membrane unit on the second feed side; providing a third membrane unit containing a third membrane having a third feed side and a third permeate side, where the third membrane is more permeable to nitrogen than to methane and is connected in series with the first membrane unit such that gas leaving the first permeate side can enter the third membrane unit on the third feed side; passing the gas stream at a first pressure into the first membrane unit at a first inlet of the first feed side; withdrawing from a first outlet of the first feed side a first residue stream (103) enriched in nitrogen compared with the gas stream; withdrawing from the first permeate side, at a second pressure lower than the first pressure, a first permeate stream depleted in nitrogen compared with the gas stream; passing the first residue stream into the second membrane unit at a second inlet of the second feed side; withdrawing from a second outlet of the second feed side, a second residue stream (105) enriched in nitrogen compared with the first residue stream enriched in nitrogen compared with the first residue stream; withdrawing from second permeate the side, at a third pressure lower than the first pressure, a second permeate stream (106) depleted in nitrogen compared with the first residue stream; passing the first permeate stream (107), at a fourth pressure into the third membrane unit at a third inlet of the third feed side; withdrawing from a third outlet of the third feed side a third residue stream depleted in nitrogen compared with the first permeate stream; and withdrawing from the third permeate side, at a fifth pressure lower than the fourth pressure, a third permeate stream enriched in nitrogen compared with the first permeate stream.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a gas stream.    Gas stream (101, 109, 110)    First residue stream (103)    Second residue stream (105)    Second permeate stream (106)    First permeate stream (107)