Operating the cooling system of a methanation reactor using an open-loop two-phase thermosiphon with evaporating water as the cooling liquid is an effective way to ensure safe operation as it does not rely on a recirculating pump. The aim of the reactor design is to co-generate methane and steam, the latter to be used in a solid-oxide electrolyzer to produce H2 to be injected in the reactor. The passive operation was analyzed to ensure a similar level of stability in the steam production compared to the active operation (i.e., with a pump). A total of 98% of the measured steam produced in passive cooling were within ±9.82% of the individual means, comparing well to ±9.5% for the active cooling operation.