Method for manufacturing a product gas and generating steam, and modular product gas-steam reactor for carrying out said method

Disclosed is a method for generating steam and manufacturing a product gas by catalytically reacting a feed gas in a reactor unit comprising a reactor tube. Said method encompasses the following steps: - a catalyst bed is conveyed through the reactor tube; - the feed gas is allowed to flow into the catalyst bed against the direction of travel of the catalyst bed; - a temperature profile is regulated along the reactor tube by thermally insulating, heating, and/or cooling regulation sections in the reactor tube; and - the waste heat generated in one of the regulation sections by the cooling action is transferred from the reactor tube to a steam generation unit. The feed gas can be syngas and especially biogas.