Reformer furnace for performing an endothermic process, comprising a reaction space, a firing space and a flue gas channel in fluid connection with the firing space; wherein the reaction space is formed by a plurality of vertical reaction tubes passing through the firing space, wherein the reaction tubes comprise in the tube interior a catalyst for converting a gaseous input material, wherein the reaction tubes are arranged in line and form one or more reaction tube rows; and wherein the firing space is formed by a plurality of refractory walls, comprises a first end face and a second end face and comprises a plurality of burners arranged at the first end face which produce flames oriented in the direction of the second end face, wherein the burners are arranged in line and form a plurality of burner rows, wherein a burner row for firing the reaction tubes is arranged parallel to a reaction tube row; and wherein the flue gas channel for withdrawing burner offgases is arranged in the region of the second end face of the firing space and runs parallel to the burner rows and outside the firing space, the flue gas channel has a transition region and a withdrawal region, wherein the flue gas channel is connected to the second end face of the firing space via the transition region and the transition region of the flue gas channel has a reduced channel diameter compared to the withdrawal region of the flue gas channel, characterized in that the flue gas channel is in the form of a one-piece construction, wherein the transition region and the withdrawal region of the flue gas channel are joined to one another with a mechanical or atomic-level join and the channel diameter of the transition region narrows from the second end face of the firing space in the direction of the withdrawal region, subsequently has a constant diameter and subsequently widens in the direction of the withdrawal region. a reaction space, a firing space and a flue gas channel in fluid connection with the firing space; wherein the reaction space is formed by a plurality of vertical reaction tubes passing through the firing space, wherein the reaction tubes comprise in the tube interior a catalyst for converting a gaseous input material, wherein the reaction tubes are arranged in line and form one or more reaction tube rows; and wherein the firing space is formed by a plurality of refractory walls, comprises a first end face and a second end face and comprises a plurality of burners arranged at the first end face which produce flames oriented in the direction of the second end face, wherein the burners are arranged in line and form a plurality of burner rows, wherein a burner row for firing the reaction tubes is arranged parallel to a reaction tube row; and wherein the flue gas channel for withdrawing burner offgases is arranged in the region of the second end face of the firing space and runs parallel to the burner rows and outside the firing space, the flue gas channel has a transition region and a withdrawal region, wherein the flue gas channel is connected to the second end face of the firing space via the transition region and the transition region of the flue gas channel has a reduced channel diameter compared to the withdrawal region of the flue gas channel, characterized in that the flue gas channel is in the form of a one-piece construction, wherein the transition region and the withdrawal region of the flue gas channel are joined to one another with a mechanical or atomic-level join and the channel diameter of the transition region narrows from the second end face of the firing space in the direction of the withdrawal region, subsequently has a constant diameter and subsequently widens in the direction of the withdrawal region.