The present study aims to understand the dynamics of particle growth inside a minichannel where evaporation from heated wet wall column generates supersaturated conditions. Such multiphase flow with phase-change is encountered in condensation particle sensors where nanoscale particles grow to micrometer size and can be measured optically. To develop condensation particle sensors that are miniscale and highly portable, we have computationally modeled the flow, heat, and mass transfer in a minichannel and determined parameters that facilitate particle growth. The mass, momentum, energy, and species conservation equations are solved, and particles are tracked and their growth through condensation is determined. Variation of thermophysical properties as a function of temperature and species concentration is incorporated for accurate determination of particle growth. The results show that the size of condensation sensors can be decreased by employing minichannels where conditions can be created, which enhance supersaturation region inside the channel where condensation occurs on the nanoparticles by heterogeneous nucleation and cause them to grow to micron sizes. The effects of inlet humidity, inlet temperature, inlet flow rate, and wall temperature on the operation of the miniscale sensor are investigated. The numerical framework provides solution to optimal working of the sensor.