In the context of using ligno cellulosic or waste feedstock for energetic applications, it is essential to produce a clean syngas able to be used in further catalytic conversion. Entrained flow gasification is a very promising reactor technology. A slagging entrained flow reactor gives the possibility to minimize difficulties encountered with ash agglomeration, corrosion, and fouling. The operation of such cooling screen walls is empirical and not well understood theoretically. This paper details a physical approach that proposes a relation between the ash composition and the optimal operation gas temperature in the reactor, based on thermodynamic calculations of the ash liquidus temperature. This approach allows the elaboration of a deterministic methodology that enables determination of either the optimum reactor operation temperature as a function of the actual ash composition or the composition and amount of additives that can be mixed with the resource in order to reach a predetermined operation temperature and slag viscosity for a given ash composition. This technique, validated at the laboratory scale, allows therefore adapting the operation parameters of an entrained flow reactor to diverse resources in order to control simultaneously the thickness of the solidified ash layer, the slag viscosity, and the heat losses.