Power-to-methane　(PtM)　is　a　strategy　for　seasonal　energy　storage　to　enable　wider　energy　share　through　the　existing　natural　gas　networks　H2O/CO2　co-electrolysis　and　methanation　reaction　can　synchronously　occur　in　a　single　tubular　solid　oxide　electrolysis　cell　(SOEC)　reactor　to　facilitate　direct　PtM.　In　this　study,　we　aim　to　intensify　the　synergy　between　geometrical　structure　and　complex　physical–chemical　processes　in　a　tubular　SOEC　reactor　for　higher　Electricity-to-CH4　efficiency.　First,　we　divided　the　reactor　into　two　parts:　Electrolysis　region　and　Methanation　reaction　(MR)　region,　and　investigated　their　respective　contribution　and　interaction.　According　to　a　self-developed　multi-physics　tubular　SOEC　model,　we　optimized　the　geometry　of　the　reactor　to　synergistically　regulate　the　field　temperature　of　the　Electrolysis　and　MR　region.　The　results　indicate　that　CH4　production　ratio　and　Electricity-to-CH4　efficiency　can　be　enhanced　through　the　lengthening　of　either　the　Electrolysis　region　or　the　MR　region.　When　fed　with　a　cathode　gas　of　75%H2O　+　20%CO2　+　5%H2　(20　mL　min−1),　an　tubular　SOEC　reactor　optimized　with　an　Electrolysis　region　of　0.10　m　and　a　MR　region　of　0.045　m　can　convert　91%　of　CO2　to　CH4　at　1.5　V　with　an　Electricity-to-CH4　efficiency　of　49%.　An　improved　synergy　between　pressure,　operating　voltage　and　gas　flow　can　significantly　improve　CH4　production　with　less　electricity.　A　tubular　SOEC　reactor　with　the　optimized　geometry　can　convert　95%　of　CO2　to　CH4　at　1.2　V　and　16　bar　with　an　Electricity-to-CH4　efficiency　of　81%,　producing　52　vol%　of　dry　CH4　at　the　cathode　outlet.　After　optimization,　tubular　SOEC　reactor　can　realize　a　more　efficient　PtM,　thus,　promote　the　combination　of　renewable　power　and　natural　gas　networks.　©　2020　Elsevier　Ltd