Process for synthesizing a product gas, comprising the steps: (a) provision of a first feed gas; (b) provision of a second feed gas; (c) combining of the first feed gas and the second feed gas to give a gas mixture A comprising first feed gas and second feed gas; (d) combining of the gas mixture A with a circulated gas mixture to give a gas mixture B comprising first feed gas, second feed gas and product gas; (e) reaction of the first feed gas and second feed gas present in gas mixture B over a catalyst to a product gas with production of a gas mixture C enriched in product gas; (f) removal of product gas from the gas mixture C enriched in product gas with production of a gas mixture D depleted in product gas; and (g) recirculation of at least part of the gas mixture D depleted in product gas as circulated gas mixture to step (d); where the volume flow of the first feed gas provided in step (a) and/or the volume flow of the second feed gas provided in step (b) and/or the volume flow of the product gas removed in step (f) is varied during the process, further comprising - the temporary reduction of the amount of product gas formed per unit time during a first time interval by reduction of the volume flow of the first feed gas provided in step (a) and/or the volume flow of the second feed gas provided in step (b) and/or the volume flow of the product gas removed in step (f); and - increasing of the amount of product gas formed per unit time during a second time interval by increasing the volume flow of the first feed gas provided in step (a) and/or the volume flow of the second feed gas provided in step (b) and/or the volume flow of the product gas removed in step (f), characterized in that - the reaction in step (e) occurs in at least a first catalyst bed and a second catalyst bed; - a first substream of the gas mixture B is fed to the first catalyst bed and the gas mixture which leaves the first catalyst bed is fed to the second catalyst bed; - a second substream of the gas mixture B is fed to the second catalyst bed with bypassing of the first catalyst bed; - the relative volume flow ratio of the first substream to the second substream is increased during the first time interval; and - the relative volume flow ratio of the first substream to the second substream is reduced again during the second time interval. (a) provision of a first feed gas; (b) provision of a second feed gas; (c) combining of the first feed gas and the second feed gas to give a gas mixture A comprising first feed gas and second feed gas; (d) combining of the gas mixture A with a circulated gas mixture to give a gas mixture B comprising first feed gas, second feed gas and product gas; (e) reaction of the first feed gas and second feed gas present in gas mixture B over a catalyst to a product gas with production of a gas mixture C enriched in product gas; (f) removal of product gas from the gas mixture C enriched in product gas with production of a gas mixture D depleted in product gas; and (g) recirculation of at least part of the gas mixture D depleted in product gas as circulated gas mixture to step (d); - the temporary reduction of the amount of product gas formed per unit time during a first time interval by reduction of the volume flow of the first feed gas provided in step (a) and/or the volume flow of the second feed gas provided in step (b) and/or the volume flow of the product gas removed in step (f); and - increasing of the amount of product gas formed per unit time during a second time interval by increasing the volume flow of the first feed gas provided in step (a) and/or the volume flow of the second feed gas provided in step (b) and/or the volume flow of the product gas removed in step (f), - the reaction in step (e) occurs in at least a first catalyst bed and a second catalyst bed; - a first substream of the gas mixture B is fed to the first catalyst bed and the gas mixture which leaves the first catalyst bed is fed to the second catalyst bed; - a second substream of the gas mixture B is fed to the second catalyst bed with bypassing of the first catalyst bed; - the relative volume flow ratio of the first substream to the second substream is increased during the first time interval; and - the relative volume flow ratio of the first substream to the second substream is reduced again during the second time interval.