A system and method to generate steam from a feed water stream does so in a liquid pool zone of a vessel as the stream comes into contact with a heating medium that is less volatile than the feed water stream. To keep the pool hot, the heating medium can be recirculated through a heater of a pump-around loop or a heater can be placed in the liquid pool. As the feed water stream is vaporized or partially vaporized, any solids or unvaporized water present in the feed water stream come out of the stream and move into the heating medium. These solids and the unvaporized water may be further removed from the heating medium in the pool or in the pump-around loop. The heat exchange surface does not contact the feed water to generate steam.