TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: One or two sets of methanation reactor are connected in series, where the first set uses water-cooling or air cooling type heat-exchanging reactor.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The method further comprises pressurized circulation process, and methanol synthesis and separation of methanol, where the pressurized circulation process comprises utilizing a part of fresh gas or all of the gas in constant pressure bypass for pressurized circulation. The methanol synthesis and methanol separation processes comprise methanation reaction to obtain synthetic natural gas, where all or a part of the fresh air is used in pressurized recycling of the gas in the circulation machine inlet end, carrying out pressurized circulation through the circulating machine, separating methanol after methanol synthesis, dividing the reaction gas into two, where one part is used as recycle gas and returning to circulation machine inlet, and the other part is allowed to flow into methanation reaction to obtain synthetic natural gas; all or part of the fresh air in the circulation machine outlet is connected with the back end of the recycling gas, separating methanol, dividing the reaction gas into two, where one part is used as recycle gas and returning to circulation machine inlet, and the other part is used to be purged into methanation reaction to obtain synthetic natural gas; or using the fresh air without pressurizing to directly synthesize methanol, and separating methanol and carrying out methanation reaction to obtain synthetic natural gas. The methanation step is carried out at less than 550 degrees C, and is carried out in a series of two steps, where a subsequent second methanation process is not required after the first methanation step if the methanation recycled gas ratio is less than 2. Preferred Components: The fresh air is divided into two parts, where one part is removed from the methanol synthesis step, and the other part is not allowed to flow into bypass stream with methanol synthesis reaction gas, and the volume ratio (N) of the fresh air in the methanol synthesis process is 0.5-1. N is equal to V(fresh air in methanol synthesis)/V(total fresh gas). The circulating air volume recycle ratio (Re) satisfies the requirement Re is less than 2, where Re is equal to V(recycled gas)/V(total fresh gas). The molar ratio of the fresh gas M1 is 2.2-2.9, where M1 is equal to (hydrogen-carbon monoxide-2 atoms of oxygen)/(carbon monoxide + carbon dioxide), and M1 is the mol content of each component. The hydrogen to carbon ratio satisfies the requirement of M2 of 2.95-3.1, where M2 is equal to (hydrogen-carbon dioxide+ethene+2 atoms of ethane)/(carbon monoxide + carbon dioxide+ ethene+ ethane), and M2 is the mol content of each component. The methanol synthesis reaction is carried out at a pressure of 2-8 MPa, where the controlled pressure increases methanol yield. The fresh gas is coke-oven gas or blast furnace gas rich in methane, which does not require gasification and carbon monoxide conversion, and can be directly desulfurized, where the methanol product is separated and reduced to obtain artificially synthesized natural gas, urban gas or liquefied natural gas after extracting hydrogen and purifying.