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.