A description in fig1 a hydrocarbon feedstock 10 is formed in a pre reformer 15 from a hydrocarbon stream 11 and steam 12 the hydrocarbon feedstock 10 is passed to the heat exchange reformer 25 where it is catalytically steam reformed and thereafter leaves the heat exchange reformer as stream 30 the primary reformed gas 30 is fed to an autothermal reformer 75 to which oxidant 80 is also supplied the primary reformed gas is partially combusted and brought towards equilibrium over reforming catalyst in the autothermal reformer 75 the hot effluent synthesis gas 90 from the autothermal reformer 75 is split into first 111 and second 112 synthesis gas streams heat is recovered from the first synthesis gas stream 111 by passing it to the heat exchange reformer 25 this first synthesis gas stream 111 is cooled by heat exchange with the gas undergoing reforming over the catalyst in the heat exchange reformer 25 the thus cooled first synthesis gas stream 111 leaves the heat exchange reformer as stream 121 the second synthesis gas stream 112 is passed through a waste heat boiler 115 to produce a cooled second synthesis gas stream 122 via heat exchange in the embodiment shown in fig1 the cooled second synthesis gas stream 122 and the cooled first synthesis gas stream 121 are not combined in fig2 a hydrocarbon feedstock 10 is formed in a pre.