TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: In the method (m2) the natural gas stream contains an acid gas component (at least 3 vol.%). The inert gas contains oxygen (less than 10 ppm). The inert purge gas contains argon, nitrogen and/or helium, and less than 10 (preferably less than 40) ppm of oxygen. In the method (m3) the inert gas contains less than 10 ppm of oxygen.    TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Apparatus: In the apparatus, the first and second oxygen concentrations differ by at least 10 ppm oxygen to inert gas. The apparatus comprises one carbon membrane having a first CO2 permeability, and another carbon membrane having a second CO2 permeability, where the first and second CO2 permeabilities differ by at least 10 Barrer. The apparatus comprises one carbon membrane having a first CO2/CH4 selectivity, and another carbon membrane having a second CO2/CH4 selectivity, where the first and second CO2/CH4 selectivities differ by at least 10.    TECHNOLOGY FOCUS - POLYMERS - Preferred Method: In the method (m1) the maximum temperature of pyrolysis is 450-800 degrees C. The precursor is heated at the maximum temperature for at least one hour. In the method (m5) the oxygen concentration is selected to increase the permeability of the carbon membrane. In the method (m5), the oxygen concentration is maintained between 1-30 ppm. The heating temperature is also selected to provide a predetermined degree of CO2 permeability. The heating temperature is 450-1000 degrees C. The process (m5) further involves performing steps (a) and (b) an additional time, where a first carbon membrane having a first predetermined degree of CO2 permeability is produced and a second carbon membrane having a second predetermined degree of CO2 permeability is produced, the first and second predetermined degrees of CO2 permeability differing by at least 10 Barrer. In the method (m6), the oxygen concentration is selected to increase the CO2/CH4 selectivity of the carbon membrane. The oxygen concentration is maintained 1-30 ppm. The oxygen concentration is maintained at 30-50 ppm. The heating temperature is also selected to provide a predetermined degree of CO2/CH4 selectivity. The heating temperature is 450-1000 degrees C. The process (m6) further involves performing all the steps, an additional time, where a first carbon membrane having a first predetermined degree of CO2/CH4 selectivity is produced and a second carbon membrane having a second predetermined degree of CO2/CH4 selectivity is produced, the first and second predetermined degrees of CO2/CH4 selectivity differing by at least 10. Preferred Components: In the method (m1) and (m4) the precursor is in the form of an asymmetric hollow polymer fiber. The polymer is a polyimide. The polymer precursor is formed from 5,5'-(2,2,2-tri fluoro-1-(trifluoromethyl)ethylidene)bis-1,3-isobenzofurandione monomer. The polymer precursor is formed from an imide reaction product of 3,3',4,4'-benzophenon tetracarboxylic dianhydride and diaminophenylidene. EXAMPLE - No suitable example is given.