NOVELTY - Fuel cell system comprises at least one reformer with at least one high-temperature fuel cell, where the reformer (1) is constructed using a material or a combination of materials having a reforming effect, and the reformer is in the immediate proximity of the high-temperature fuel cell and/or encloses the high-temperature fuel cell (2).    USE - As fuel cell system (claimed).    ADVANTAGE - The fuel system is suitable for mobile or portable applications. The system comprises a compact reformer that uses the generated or consumed heat as efficient as possible, and uses the heat immediately in situ in other heat-consuming or heat-producing components such as fuel cells, heat exchangers, and burners, thus compensates for the thermal losses, in particular in case of smaller systems with high-temperature fuel cells, thus minimizes the system costs and increases the efficiency. The reforming material of the reformer is located in the immediate proximity of an electrode of at least one high-temperature fuel cell. At least one fuel cell of the system can also be directly enclosed by the reforming material or a combination of such material. Thus, an intensive exchange of thermal energy through radiation, convection and thermal conduction can be achieved. While the reforming reaction takes place, the reforming material of the reformer can emit heat and thus can be used to heat the fuel cells and the system, or the material can consume heat and thus contribute to the cooling of the fuel cells and the system. The reforming material can also have a neutral effect on the system's thermal balance. The fuel cell system offers the option to utilize the supplied amount per time unit of at least one (at least under reforming conditions) gaseous reactant to control the temperature and/or the performance and/or efficiency of the system or parts of the system. In case of a partial oxidation reformer, an increased oxygen/fuel ratio can be used to start up the system, i.e. a larger amount of air, at a constant supply of fuel, which leads to a larger amount of complete oxidation inside the reformer and which causes a higher heat production. By selecting the appropriate fuel and/or catalyst, the starting of an exothermic reaction, and with that an increase in heat, is already possible at low temperatures. The option to select an additional burner for heating the system to its operating temperature is also available. Once the operating temperature has been reached, the oxygen/carbon ration is reduced, which in turn will reduce the heat production and simultaneously increases the system's efficiency. In order to cool down the system, generally a smaller quantity of reactant is supplied; if necessary, the oxygen/carbon ratio is increased again in order to prevent or reduce the undesirable deposition of carbon inside the system. Based on the high resistance against temperature changes of the micro-tubular fuel cells, which are preferably used, it is possible to cool down the reformer very rapidly and complex control systems are unnecessary. Since the system uses micro-tubular cells, temperature fluctuations inside the reformer during the operating mode do not pose any difficulties. The resistance to variations of temperature allows for a very compact design, which enables the reformer to be cooled down as well, by means of increased air supply in the cathode segment of the used fuel cells, which in turn reduces the number of valves, regulator components and other peripheral devices.    DETAILED DESCRIPTION - Fuel cell system comprises at least one reformer with at least one high-temperature fuel cell, where the reformer (1) is constructed using a material or a combination of materials having a reforming effect, and the reformer is in the immediate proximity of the high-temperature fuel cell and/or encloses the high-temperature fuel cell (2). In the fuel cell system, at least one additional material/additive exhibiting or promoting a higher catalytical activity is contained in and/or coated on the material that has a reforming effect. The fuel cell system comprises at least one heat exchanger and/or afterburner and/or peltier element and/or additional reformer, arranged in the immediate proximity of the reformer. The reformers are electrically conductive.    DESCRIPTION OF DRAWING(S) - The figure shows a view of an integrated reformer and additional gas channels.    Reformer (1)    High-temperature fuel cell (2)    Gas channels (6)