NOVELTY - Process of separation of carbon dioxide from methane in a gas stream (100) involves three membrane separation stages which are arranged so that the permeate stream from the first stage forms the feed to the second stage and the permeate stream from the second stage forms the feed to the third stage. The first permeate stream (105) is compressed before it is passed as feed to the second stage; the second permeate stream (112) is not compressed before it is passed as feed to the third stage.    USE - For separating carbon dioxide from methane in a gas stream (claimed) from natural gas wells and landfills.    ADVANTAGE - The process can treat gas streams with very high carbon dioxide concentrations and it enables to treat streams in which carbon dioxide is a minor component, such as less than 40% of the stream, and most preferably is present in a concentration below about 20%. The membranes used in the three stages are selective in favor of carbon dioxide over methane. The membranes of each stage provide a selectivity for carbon dioxide over methane of at least 5 under the operating conditions of the process. The membranes provide a permeance for methane of at least about 5 (preferably 10) gpu, and a permeance for carbon dioxide of at least 100 gpu under the process operating conditions. Because the membranes used in each stage preferentially reject methane, the residue streams from all three stages are enriched in methane compared with the feed streams to the respective stages. Very good performance, with a methane product of acceptable purity and recovery, can be obtained by adding a third membrane stage and operating that stage without compression. The processes achieve at least about 70% methane recovery. The ability of the third stage to improve methane recovery without necessitating the use of a second compressor is beneficial both in terms of the technical performance of the system and the economics of gas treatment. The process is operated with only one set of rotating equipment, thereby decreasing downtime for maintenance or repairs.    DETAILED DESCRIPTION - Process for separating carbon dioxide from methane in a gas stream involves: (a) providing a first membrane (103), having a first feed side and a first permeate side and being selective for carbon dioxide over methane; (b) passing the gas stream as a first feed stream at a first pressure of at least about 400 psia across the first feed side; (c) withdrawing from the first feed side a first residue stream depleted in carbon dioxide compared with the gas stream; (d) withdrawing from the first permeate side a first permeate stream enriched in carbon dioxide compared with the gas stream; (e) compressing the first permeate stream; (f) providing a second membrane (110) having a second feed side and a second permeate side and being selective for carbon dioxide over methane; (g) passing the compressed first permeate stream as a second feed stream across the second feed side; (h) withdrawing from the second feed side a second residue stream depleted in carbon dioxide compared with the second feed stream; (i) withdrawing from the second permeate side a second permeate stream enriched in carbon dioxide compared with the second feed stream; (j) recirculating the second residue stream to step (b); (k) providing a third membrane (114) having a third feed side and a third permeate side and being selective for carbon dioxide over methane; (l) passing the second permeate stream as a third feed stream without compression across the third feed side; (m) withdrawing from the third feed side a third residue stream depleted in carbon dioxide compared with the third feed stream; (n) withdrawing from the third permeate side a third permeate stream enriched in carbon dioxide compared with the third feed stream; (o) recirculating the third residue stream to step (e).    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of process of separation of carbon dioxide from methane in a gas stream.    gas stream (100)    first membrane (103)    residue stream (104)    second membrane (110)    third membrane. (114)