Heat exchanger, in particular waste gas heat exchanger

The heat exchanger (1) has a channel which is cooled by exhaust gas (2) and a fluid. Two partial heat exchanger units (4,5) are formed, where one partial heat exchanger unit has a medium (6) which is designed to dehumidify the exhaust gas. The latter heat exchanger unit is arranged below the former heat exchanger unit. An independent claim is also included for a method for operating an exhaust gas system of a motor vehicle.