A heat exchanger for cooling cracked gas has a nest of heat-exchange tubes (1) that the gas flows through and that are secured at the ends in bases (2 and 3) and surrounded by a jacket (4) that demarcates in conjunction with the bases a chamber occupied by evaporating water. Each heat-exchange tube has two sections (10 and 11) that differ in diameter and communicate through a cone (12). The section (11) with the longest diameter is at the emerging-gas end of the heat-exchange tube.