Power to Synthetic-Natural-Gas (SNG) technology consists of two main steps: water electrolysis and methanation; the primary energy input is usually surplus power from renewable energy sources, while the electrolytic hydrogen and carbon oxides from different COx sources are converted into methane that can be fed in the natural gas grid. We focus on methanation technology, where the main criteria are the complexity of process setup and reactor sizes to achieve production and SNG quality for gas-grid injection. The processes are simulated using a plug-flow model for the reactors and a pseudo-homogeneous kinetic law describing the reaction of CO2 (that is rate limiting). The results show that feeding biogas or syngas (instead of CO2) for methanation has remarkable effects regarding the operation and design of the processes; it is concluded that Power-to-SNG technologies that use methane rich streams are favorable in terms of biogas upgrading, H2 requirements, reactor volumes and process simplicity, as far as these resources are available: e.g., using a typical composition (60% CH4) the required inputs are 0.96 kmol of biogas, 1.54 kmol of H2 and 0.26 m3 of reactors (two adiabatic beds with recirculation, R/F = 0.695) per kmol/min of pipeline quality dry gas product (95% CH4), which means 60% hydrogen saving, less than 26% reaction volumes and near 62% reduction of process throughput, when compared to the methanation process that uses pure CO2; conversion of syngas can be also favorable, but it requires high recirculation due to the large proportions of COx; e.g. for syngas (47.3%H2-25.9%CO-17.2%CO2-9.6%CH4), the required values mean a 53% hydrogen saving and less than 25% reaction volumes, but only 11% reduction of process throughput. •Comprehensive analysis of Power-to-SNG technologies using different COx sources.•Methane rich feedstocks are favorable in terms of biogas or syngas upgrading.•Process is optimized in two adiabatic beds with recirculation and water condensation.•The product fits with requirements for injection into the natural gas grid (95% CH4).