Combined removal of both ammonia from an ammonia-containing waste gas and nitrogen oxides from a nitrogen oxide-containing waste gas in a combined ammonia/urea synthesis plant is accomplished by mixing the gases and employing one or both of selective non-catalytic reduction at a temperature of 850° C. to 1100° C. or selective catalytic reduction at a temperature of 150° C. to 550° C., in which the ammonia and the nitrogen oxides react with one another to give nitrogen and water, the ammonia-containing waste gas derived from a low-pressure and/or atmospheric absorber of the urea synthesis plant, and the nitrogen oxide-containing waste gas derived from a flue gas duct of a primary reformer of the ammonia synthesis plant, both the ammonia and the nitrogen oxides of the mixed waste gas flows being depleted simultaneously during the same process step.