Some of the features of the dynamic flowsheeting package, POLYRED, are described. To demonstrate some of the capabilities of the package, POLYRED is used to formulate models and to estimate model parameters from both batch and continuous process data. Examples include the complications of imperfect micromixing which require mixing models. Both kinetic and micromixing parameters are estimated from data. Results for a low-density polyethylene autoclave reactor are provided as an illustration.