NOVELTY - Method without a circulation loop for producing synthetic natural gas by complete methanation of synthesis gas comprises: carrying out conversion reaction on one part of crude synthesis gas to obtain hydrogen-rich crude synthesis gas, and direct cooling on the other part of the crude synthesis gas, and separating liquid water to obtain carbon-rich crude synthesis gas; purifying the crude hydrogen-rich synthesis gas and carbon-rich crude synthesis; and carrying out methanation reaction of purified hydrogen-rich synthesis gas or carbon-rich synthesis gas as a reaction stream.    USE - The method is useful for producing synthetic natural gas without a circulation loop by complete methanation of synthesis gas (claimed).    ADVANTAGE - The method has saves investment and energy consumption and is easy to control and operate.    DETAILED DESCRIPTION - Method without a circulation loop for producing synthetic natural gas by complete methanation of synthesis gas comprises (i) passing crude synthesis gas with carbon monoxide from the upstream device into conversion means divided into two parts, carrying out conversion reaction on one part of crude synthesis gas to obtain hydrogen-rich crude synthesis gas, and direct cooling on the other part of the crude synthesis gas, and separating liquid water to obtain carbon-rich crude synthesis gas; (ii) purifying the crude hydrogen-rich synthesis gas and carbon-rich crude synthesis gas in a purification device, and removing impurity gases and small amount of water; (iii) passing the purified hydrogen-rich synthesis gas or carbon-rich synthesis gas as a reaction stream into a methanation reactor for methanation reaction, using carbon-rich synthesis gas or the hydrogen-rich synthesis gas as a control stream for controlling the reaction depth of other stages of methanation reactors, and taking the reaction product out of the first-stage methanation reactor, cooling and subjecting to supplementation of part of the control steam and then entering into the next-stage methanation reactor until all of the control stream is supplemented into the reaction stream before the last-stage methanation reactor; and (iv) passing the reaction product which is rich in methane into a synthetic natural gas post-treatment unit, and performing drying and compressing.