Abstract: Product gas resulting from the partial oxidation of finely divided fuel in a reactor is discharged as a stream into a tube leading downwardly into a vessel containing a water bath in which the lower end of the tube is immersed. Liquid slag produced during the partial oxidation is also discharged as a solid stream within and surrounded by the stream of product gas. The tube is cooled by circulating cooling water upwardly through an annular clearance in the tube which is discharged from the annular clearance at the upper end of the tube for gravity descent as a liquid film along the inside of the tube. The slag becomes cooled and granulated on contact with the water bath and is discharged from the vessel in granulated condition. The gas escapes from the lower open tube end, travels upwardly through the water bath to become cooled thereby, and is then discharged from the vessel in cooled condition.