The exhaust gas after-treatment system of an engine configured as a gas engine or as a dual-fuel engine includes a catalyst, that can be flowed through by exhaust gas, a control tube extending through a recess in the catalyst, which control tube is movable relative to the catalyst and is flowable through by exhaust gas, and an actuator which is equipped to move the control tube relative to the catalyst dependent on at least one operating condition of the engine and/or at least one operating condition of the exhaust gas after-treatment system such that in a first relative position of the control tube relative to the catalyst, the catalyst can be flowed through by exhaust gas but not the control tube, and in a second relative position of the control tube relative to the catalyst, the control tube can be flowed through by the exhaust gas but not the catalyst.