A judgment model for adhesion of a particle was developed in order to predict the adhesion phenomenon between solid particles and the furnace wall in a coal gasifier, and it was used to predict the phenomena due to impaction of particles. The Voigt-Kelvin model, which is able to express realistically the contact between spherical particles, was adapted for use as the judgment model. If the particle velocity after the impaction was smaller than a threshold value, the particle adhered; otherwise, it did not. The coal combustion in the coal combustor in the runup to the coal gasification reaction in the coal gasifier was numerically analyzed, and calculated velocity gave the trajectory of the particles. The adhesion judgment model was applied to predict the deposition occurring through the impaction of particles with the wall of the furnace. When the van der Waals force was adopted as the only adhesive force acting between the particle and the wall, the adhesive mass of particles calculated by the model developed in this study differed from the one calculated by the traditional model based on ash viscosity.