The present study analyzes an innovative method for the disposal and effective exploitation of sewage sludges, based on the integration of oxy-steam co-gasification in downdraft gasifier with wood chips and novel methanization plant for the final production of added value bio-methane. The analysis of the co-gasification process with oxygen-steam, as well as the following one of methanization, was performed numerically by creating a model of the overall plant within the Aspen Plus commercial code. The gasification model was validated with the experimental results obtained with a full-scale air gasifier fed with sewage sludge and wood chips. During the experimental activity, no reactor blockage nor ash discharge problems were recorded until the sewage sludge percentage (by mass) did not overcome roughly 30%. The numerical results show that the oxy-steam co-gasification process has a high energy conversion efficiency that reaches roughly 80% and the overall methane yield of the process is about 20% (by mass) of the feedstock utilized. The water consumption, which is higher than 1.0 kg/kg methane, and the electricity demand, around 0.70 kWh/kg methane, result critical issues for methane production. Therefore, the present research represents a further insight into the possibility to co-gasify biomass with sewage sludge, giving new information on this alternative route for waste exploitation both experimentally and numerically with the production of bio-methane.