Reformer and method for producing hydrogen and steam where steam is used for steam-assisted extraction of heavy hydrocarbons. Steam is injected into a hydrocarbon-containing reservoir. Hydrocarbons are extracted from the reservoir along with produced water. Hydrogen is produced in a catalytic steam hydrocarbon reformer. Combustion product gas from the reformer is used to generate wet steam in a once-through steam generator from produced water recycled from the reservoir. The wet steam is used for the steam-assisted extraction of heavy hydrocarbons. The reformer has a heat exchanger section including a first heat exchanger and a second heat exchanger downstream of the first heat exchanger. The second heat exchanger is suitable for processing the produced water by once-through steam generation and is suitable for mechanical cleaning. The reformer also has a first exhaust downstream of the second heat exchanger and a closeable second exhaust downstream of the first heat exchanger.