TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Conditions: The inlet temperature of methanation catalyst layer in the first period is 330-240 degrees C; and the outlet temperature of the catalyst layer is kept within 270-180 degrees C The inlet temperature of the methanation catalyst layer in the second period is 220-140 degrees C; and the outlet temperature of the catalyst layer is kept within 180-110 degrees C. Preferred Process: The temperature of methanation catalyst layer in the first period is gradually reduced along the axial line of the reaction pipe by cooling the reactor. The temperature of the methanation catalyst layer in the second period is gradually reduced along the axial line of the reaction pipe by cooling the reactor. The temperature of the material is reduced after the first period and before the second period of the methanation. The temperature of the material is reduced after the first period and before the second period of the methanation. A heat exchanger is arranged between the methanator for the first period and the methanator for the second period, to reduce the temperature of the outlet material of the methanator for the first period. Preferred Composition: The reformed hydrogen contains CO, CO2 and methane (CH4), whose percentage compositions in volume are respectively 0.4-2.0%, 15-30% and 25%, and the rest is hydrogen gas (H2). Preferred Parameters: The first methanation catalyst is a high-selectivity one; when the gaseous hourly space velocity (GHSV) of the reaction mass at the inlet is 2500/hour, the percentage composition in volume of the CO in the reaction mass at the inlet is 2%, and the percentage composition in volume of the CO2 is 20%, the inlet temperature of the catalyst layer is greater than or equal to 240 degrees C when the conversion rate of the CO reaches 95%; while the inlet temperature of the catalyst layer is greater than or equal to 300 degrees C when the conversion rate of the CO2 reaches 5%. The second methanation catalyst is a high-activity one; when the gaseous hourly space velocity GHSV of the reaction mass at the inlet is 2500/hour, the percentage composition in volume of the CO in the reaction mass at the inlet is 2%, and the percentage composition in volume of the CO2 is 20%, the inlet temperature of the catalyst layer is greater than or equal to 220 degrees C when the conversion rate of the CO reaches 95%; while the inlet temperature of the catalyst layer is greater than or equal to 200 degrees C when the conversion rate of the CO2 reaches 5%.