A method of operating a heat exchange device downstream of a gasification reactor for the partial combustion of a carbonaceous feed for the production of synthetic gas. The produced synthetic gas flows through the heat exchange device with a flow velocity which is adjusted as a function of the composition and/or particle size of fouling components carried by the synthetic gas, in particular fly ash. A heat exchange device comprising a channel surrounding one or more heat exchange surfaces and having an adjustable flow-through capacity. The heat exchange surfaces can, e.g., be cylindrical and be coaxially nested, the inner heat exchange surface defining an inner channel with one or more closing members, which are moveable between a closing position and an opening position.