A reactor for carrying out a chemical reaction in a three phase slurry system providing a horizontal reaction vessel with a cross sectional area which is dependent on the vessel length, vessel diameter, and axial position. The vessel has a gas inlet at or near the bottom of the reaction vessel and a gas distributor. The gas product exits the vessel by conduit means at or near the top of the reaction vessel. The vessel includes a plurality of horizontal cooling coils to provide a cooling medium to the slurry. In the reaction vessel, the synthesis gas has an average linear velocity which is a function of the vessel cross sectional area.