Two supply chains for biomethane are here analyzed and modeled: gasification of short rotation forestry (SRF) poplar wood chips and anaerobic digestion of two species of microalgae, Chlorella vulgaris, and Nannochloropsis gaditana. Mass and energy balances are carried out along the whole systems, from the cultivation step to the stages of upgrading and injection into gas grid. A simulation model is employed to obtain gasification and upgrading parameters, energy requirements, and CO2 emission. Harvesting of microalgae requires high inputs of energy and fertilizers, while syngas upgrading is more demanding than biogas treatment, as high volumes of water and CO2 must be separated. Yields of the supply chains are very different: 7:1 biomethane:syngas ratio and 2:1 biomethane:biogas ratio have been estimated. Both of the supply chains could be optimized by using heat recovery methods. The highest removal of CO2 and steam can be attained through absorption by triethylene and monoethanolamine solution, respectively.