Process for the production of substitute natural gas (SNG) by the methanation of a synthesis gas derived from the gasification of a carbonaceous material together with water gas shift and carbon dioxide removal thereby producing a synthesis gas with a molar ratio (H2−CO2)/(CO+CO2) greater than 3.00. At the same time, a gas with a molar ratio (H2−CO2)/(CO+CO2) lower than 3.00 is added to the methanation section. The final product (SNG) is of constant high quality without excess of carbon dioxide and hydrogen.