A METHOD FOR EFFECTING HIGH YIELD HETEROGENEOUS EXOTHERMIC SYNTHESIS REACTIONS A method for effecting high yield heterogeneous exothermic synthesis reactions, of the type comprising the steps of: feeding gaseous reagents to a synthesis reactor comprising a shell (2) wherein there are supported, superimposed and in mutually spaced relationship, at least a fist catalytic bed (12) extending in an upper portion (2a) of said shell (2), at least a second catalytic bed (13) and a lowermost catalytic bed (14) extending in a lower portion (2b) of said shell (2); reacting said gaseous reagents in said catalytic beds (12, 13, 14); withdrawing from the synthesis reactor the reaction products coming from said lowermost catalytic bed (14); characterised in that it comprises the step of: causing a reaction mixture to flow in said lowermost catalytic bed (14) through a reaction volume smaller than the reaction volume of said second catalytic bed (12), and comprising a catalyst having a reaction activity higher than the activity of the catalyst loaded in the other catalytic beds (12, 13).