Method for purifying a foreign-substance-laden gas flow of the foreign substance, with the gas flow being conducted through a heat exchanger and being placed in thermal contact with a cooling medium in order to freeze and/or condense the foreign substance out, characterized in that the purification takes place in only one tube heat exchanger, through the interior space of which the gas flow is conducted from a first end region to a second end region, and here is cooled by way of contact with a first group of tubes which are traversed by the coolant, and in that the gas flow, in the second end region, is directly deflected again and is conducted back through the interior space of the heat exchanger to the first end region through a second group of tubes while undergoing an exchange of heat with the gas flow flowing into the interior space.