The performance of a novel biofilm plug flow reactor containing a mixed anaerobic microbial culture was investigated for the conversion of CO2/H2 to CH4. Unlike conventional gas-liquid contactors that depend on agitation, gas diffusion was decoupled from power consumption for mixing by increasing the gas phase inside the reaction space whilst increasing the gas residence time. The mixed mesophilic culture exhibited good biofilm formation and metabolic activity. Within 82 days of operation, 99% and 90% CH4 conversion efficiencies were achieved at total gas throughputs of 100 and 150 v/v/d, respectively. At a gas input rate of 230 v/v/d, methane evolution rates reached 40 v/v/d, which are the highest to date achieved by fixed film biomethanation systems. Significant gas transfer related parasitic energy savings can be achieved when using the novel plug flow design as compared to a CSTR. The results and modelling parameters of the study can aid the development of high rate, low parasitic energy biological methanation technologies for biogas upgrading and renewable power conversion and storage systems. The study has also established a reactor system which has the potential of accelerating biotechnology developments and deployment of other novel C1 gas routes to low carbon products.