Abstract: Shell-and-tube transfer line heat exchangers having heat exchange tubes contained within an outer shell and a primary tubesheet, to minimize inlet end fouling, are disclosed. The exchanger includes: a secondary porous tubesheet, through which a reactive gas, e.g., an oxidizing gas such as air, oxygen, steam or mixtures thereof or a reducing gas such as hydrogen, carbon monoxide or mixtures thereof, can diffuse in amounts sufficient to react with coke deposits on the porous tubesheets process gas inlet side; the porous tubesheet is positioned preferably, but not necessarily, substantially parallel to the heat exchanger's process gas inlet end primary tubesheet nearer the inlet ends of the heat exchange tubes than the primary tubesheet and is, like the primary tubesheet, perforated by the heat exchange tubes, thus creating, with the heat exchanger's outer shell and the primary tubesheet, an enclosed space, and at least one gas inlet communicating with the enclosed space through which the reactive gas is fed.