The present invention relates to a process and plant for producing a hydrogen- and carbon oxides-having synthesis gas by partial oxidation of a carbonaceous feedstock having nitrogen compounds. The invention takes advantage of the different solubility of NH3 and CO2 as the synthesis gas stream is cooled. Water is condensed continuously throughout the cooling of the synthesis gas stream. NH3 preferentially dissolves or condenses into the liquid phase during cooling, but at relatively high temperatures, above about 80 to 100° C. Further condensation at lower temperatures then preferentially dissolves or condenses CO2 into the liquid phase. Thus, the formation of ammonium salts during cooling of the synthesis gas is prevented.