Two types of methane-hydrogen mixed gas production systems with SOEC were designed to calculate syngas compositions and system efficiencies. i) System A produces the syngas with CH4 and H2 in methanation reactors based on the Fischer-Tropsch synthesis after co-electrolysis for SOEC, ii) System B produces the syngas with CH4 and H2 in methanation reactors using Sabatier reaction after steam electrolysis for SOEC. Efficiencies were 86.5% (HHV) for System A and 84.9% (HHV) for System B. The gas compositions of CH4, H2, CO2 and H2O were 74.1%, 24.8%, 0.2% and 0.9% for System A, and 69.4%, 29.4%, 0.4% and 0.9% for System B. The maximum additions for Syngas A and Syngas B were and 51.4% and 46.4% for 13A without a heating value limitation, 5.5% and 5.0% for 13A with a heating value limitation of 44-45 MJ/Nm3, and 13.8% and 12.5% for 13A with a heating value limitation of 43-46 MJ/Nm3, respectively. Quantities of the syngases suppling to the city gas pipelines in Japan in 2030 were calculated and recycling quantities of CO2 were calculated for the systems. Utilizations of renewable energy for the mixed gases production were 19.6-196% and 18.8-173% in System A and System B,respectively. The recycling quantities of CO2 were 2.57-2.68% of the total released quantities in 2013, in Japan.