The paper presents the main results of thermal-hydraulic characteristics’ investigation of steam methane reformer with radial flow of the gas mixture in the annular channel filled with granular catalyst bed. The mixture (CH 4 + H 2O) heating and the endothermic reforming reaction warmth providing is carried out within the inner system of vertical tube bundles with helium coolant, which transfers heat from the high temperature gas-cooled nuclear reactor (HTGR). Given are the simple criterial relations describing gas heat exchange, in the conditions of confusor-diffuser flow, with the layer elements and the tube bundles with gas-cooled (helium) located in the layer. The main design parameters and thermal-hydraulic characteristics of the natural gas converter are given as an example of the effectiveness of the proposed scheme.