Energy plans of many countries anticipate an increased use of biomethane for energy supply, i.e., in power and heat production as well as in the transport sector. Existing infrastructure of natural gas storage, supply and application provides a good platform for transfer to biomethane utilization on a larger scale. Upgrade of biomass-derived synthesis gas, originating from different sources, to the quality of natural gas (SNG - Synthesis Natural Gas) via methanation process for further injection into the natural gas grid is one key element of the biomethane system. Maximisation of efficiency of the methanation process is of critical importance in order to make the biomethane technology viable for wider application. The aim of the study was to improve efficiency of methanation process by finding the optimum temperatures and pressure. Theoretical modelling of adiabatic and isothermal methanation processes by using thermodynamic equilibrium calculations was employed as a method for the study. The results show the impact of temperature and pressure changes on the overall efficiency of the methane production. It can be concluded from the study that knowledge about relation between temperature, pressure and the efficiency of the methanation process gives a possibility to optimize the process under various biomass synthesized gas input conditions.