The present invention relates to method for mixing gaseous flows at different temperatures. In particular, the present invention relates to heterogeneous exothermic synthesis reactors of the type including a plurality of superimposed catalytic beds wherein at least one of the beds is provided with a gas permeable wall for outlet of a hot gas flow. This method includes the steps of supplying gaseous reagents to at least one bed; reacting the gaseous reagents in that bed; and collecting a hot reaction gas flow leaving the gas permeable wall in an air space defined between the bed and a baffle extending below and parallel thereto. Further steps require directing the hot gas flow in the air space radially outward to an annular opening of constant thickness defined between the baffle and a side wall supporting the bed, wherein the ratio of the width of the annular opening and the thickness of the air space being between 0.2 and 1. Then, hot gas flow is discharged from air space through the annular opening and is subjected to a pressure drop of predetermined magnitude so as to uniform its flow rate. Finally, a cooling gas flow is injected into hot gas flow thus obtained, wherein cooling gas flow is provided at predetermined velocity from a perforated distributor supported below baffle, which is at a predetermined distance from the annular opening.