A process and a plant for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, wherein the H2/CO molar ratio of the product gases can be varied within a wide range. This is achieved in that at least a part of a methane-rich gas obtained during the fractionation of the raw synthesis gas is admixed to the feed gas mixture, and that in the alternative at least a part of the H2 product gas and/or a fraction of a hydrogen-rich gas increased with respect to the normal operation of the process is admixed to the heating gas mixture, in order to lower the H2/CO ratio, or at least a part of the CO product gas and/or a fraction of a carbon monoxide-rich gas increased with respect to the normal operation of the process is admixed to the heating gas mixture, in order to increase the H2/CO ratio.