NOVELTY - Production of synthesis gas comprises subjecting a first part of a gaseous hydrocarbon stream (15) to an endothermic reaction; subjecting a second part of the gaseous hydrocarbon stream (25) to an exothermic reaction; and transferring thermal energy from the exothermic reaction to the endothermic reaction.    USE - The process is for production of synthesis gas for liquid fuels.    ADVANTAGE - The energy consumption is relatively low, such that a reactor energy efficiency (lower heating value of the product gas divided by the lower heating value of the feed stream) of 95% or even larger con be obtained.    DETAILED DESCRIPTION - Production of synthesis gas comprises providing a gaseous hydrocarbon stream; subjecting a first part of the gaseous hydrocarbon stream to an endothermic reaction producing a first gaseous reaction product (17) containing hydrogen; subjecting a second part of the gaseous hydrocarbon stream to an exothermic reaction, producing a second gaseous reaction product containing hydrogen; transferring thermal energy from the exothermic reaction to the endothermic reaction; and combining the first and second gaseous reaction products, forming a gaseous reaction products mixture.    INDEPENDENT CLAIMS are also included for:    (1) a reactor comprising a first and a second section (10, 20), and including an inlet (10a) to the first section for providing a first part of a gaseous hydrocarbon stream and a first gas (16) to the first section, an outlet (10b) from the first section for a first gaseous reaction product, an inlet to the second section for providing a second part of a gaseous hydrocarbon stream and a second gas (26) to the second section, and an outlet from the second section for a second gaseous reaction product; a third section connected with the outlet of the first section and with the outlet of the second section, and arranged such that the gaseous reaction products from the first and second sections mix in the third section, forming a gaseous reaction products mixture, where first, second, and third sections and the outlets are arranged in the reactor, such that the gaseous reaction products mixture of the first and second gaseous reaction products can flow along the first section; and a reactor outlet from third section for the gaseous reaction products mixture; and    (2) a plant for providing gas to liquids products comprising the reactor, and a gas to liquids (GTL) plant.    DESCRIPTION OF DRAWING(S) - The figure schematically depicts a reactor.    First and a second section (10, 20)    Inlet (10a)    Outlet (10b)    First part of a gaseous hydrocarbon stream (15)    First gas (16)    First gaseous reaction product (17)    Second part of the gaseous hydrocarbon stream (25)    Second gas (26)