A reduced-order model (ROM) is developed to capture conjugate aero-thermal physics of effusion cooling in an entire micro-turbine vane/blade. The model considers a single-wall effusion scheme with internal boundary layer flow between the shell and an inner core. Coolant is supplied inside the leading edge and spread to both suction and pressure sides. The compound effect of multiple effusion rows is used to calculate spanwise-averaged cooling effectiveness. Metal temperature is modeled both in streamwise and shell thickness directions. The development of the model and a number of numerical/experimental validation cases are presented in detail in Part I. Part II of the work is geared toward the application of this method to skin cooling of a turbine airfoil by single-wall effusion. The reduced-order model, with all its subroutines functioning together, is validated against a higher fidelity 3D Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) solution. It is shown that the model can predict the main features of the combined internal and effusion cooling in gas-turbine blades at a computational cost which is 105 times lower than RANS (∼1 month with 700M elements and 24 modern Xeon Cores) on a whole turbine vane/blade. Due to this great advantage in speed, a design optimization is then accomplished toward minimizing coolant flow rate while keeping thermal gradients and temperature of the solid within acceptable levels. Implementing this scheme on a typical micro-gas-turbine vane, optimal distributions of the effusion-hole pitch and diameter are found within a given set of constraints. This preliminary design tool potentially enables wider and more efficient usage of effusion cooling in turbine vanes/blades.