Display omitted •A 2D heterogeneous transient model for multi-tubular methanol reactor is developed.•The model is used to simulate the operations of a biogas-to-methanol process.•The flexibility of a multi-tubular reactor is shown in the presence of load changes.•A sensitivity analysis is conducted on reactor inlet flowrate and composition.•The effects of tube length and gas hourly space velocity are assessed. The present modelling work studies the steady state and transient performances of a multi–tubular fixed bed reactor for methanol production from biogas. The modelled system comprises a methanol synthesis reactor, a flash unit, and accounts for the unconverted gas recycle. The novel concept herein proposed and assessed is the possibility to run a multi–tubular methanol synthesis reactor flexibly, i.e., using the carbon dioxide present in biogas, together with renewable H2, to increase methanol productivity, only when this is economically convenient. Accordingly, the methanol synthesis multi–tubular reactor under study is run in two operating conditions: when the cost of green hydrogen is high, the excess of CO2 present in biogas is vented and the reactor is fed with CO2-lean syngas only; conversely, in the presence of affordable renewable H2, CO2 and green H2 are cofed to the reactor. This study focuses in particular on the impacts of the two working conditions on the methanol productivity, as well as on temperature profiles and transient behaviour of the reactor. The effects of tube length and number are also simulated. The study shows that the multi–tubular reactor design well manages both operating conditions and that the steady-state conditions can be reached in a few hours after the switch from one condition to the other. In view of a small scale process, simulations show that it is better to reduce the tube number rather than their length, as the latter choice would lead to unacceptable temperature hotspots, due to a worsening of convective heat transfer.