In this study, the integration between anaerobic digestion and Power-to-Gas system is proposed and numerically investigated. The rationale behind the proposed layout is that direct conversion of biogas CO2 content to CH4 without its prior separation increases biomethane yield and reduces to nearly zero carbon dioxide emissions from biogas cleaning/upgrading process. Detailed design of a methanation unit composed of multistage adiabatic reactors is presented by combining a thermodynamic equilibrium analysis carried out by means of the Aspen plus(TM) software and process simulations at steady state (including kinetic modelling) implemented in MATLAB. The performances of the proposed layout are benchmarked against those of conventional biogas upgrading techniques in terms of methane losses and purity and CO2 recovery. The proposed system not only equals conventional technologies with respect to produced methane purity (95%) but also outperforms them with respect to CH4 losses (~0%) and CO2 recovery (>97%).