A coal liquefaction process wherein a feed coal-solvent slurry is preheated in a thoroughly back-mixed preheater zone and then passed to a dissolver zone. Hot hydrogen is introduced to the process downstream from the preheater zone at or in advance of the dissolver zone and exothermic hydrocracking reactions occur in the dissolver zone. A dissolver zone effluent stream is passed through a vapor-liquid separator and separated hot dissolver vapor at process pressure is vented through the preheater zone wherein it is quenched to provide direct transfer to the preheater zone of the exothermic heat generated in the dissolver zone. The preheater zone is continuously vented to provide rapid removal of cooled vapor from the preheater zone and from the process, advantageously leaving an accumulation of liquid in the preheater zone boiling between the temperatures of the vapor-liquid separator and the preheater zone at process pressure. Venting of hot vapor generated in the exothermic dissolver zone through the preheater zone to accomplish direct internal heat exchange within the process requires continuous venting of cooled vapor from the preheater zone independently of removal of slurry from the preheater zone. Such independent venting of the preheating zone is feasible because of the introduction of hydrogen to the process downstream from the preheating zone.