The invention discloses a process method of producing and synthesizing natural gas by gasifying garbage and solves the problem that there is no natural gas producing and synthesizing process based on the garbage gasification technology up to now. The process method includes the steps of garbage gasification, gas purification, and methanation. The garbage gasification step includes a garbage feeding and high-temperature gasification section, a coarse synthetic gas dust removal and heat recovery section, a coarse synthetic gas chilling and washing section, and a high-temperature slag chilling and slag removal section; the gas purification step includes a COS hydrolysis and normal pressure desulfurization section, an induced draft fan and gas holder storage section, a synthetic gas compression section, a CO shift conversion section and an acidic gas removal section; and the methanation step includes a methanation section and a natural purifying section. The method finally produces the qualified natural gas that satisfies requirement in national quality standard. The method is simple, has high energy utilization rate and good recovery effect, and is energy-saving, emission-reducing and environment-friendly.