A comprehensive data reconciliation package for an industrial pyrolysis reactor based on simplified non-linear mass and energy balances was developed. Both successive Linearisation and Successive Quadratic Programming (SQP) methods were used to solve the data reconciliation problem. The package was tested using plant data collected at regular interval over a full generation/regeneration cycle of the reactor. Presence of gross errors in the data set was determined using a global test, and these were detected and rectified using Serial Elimination analysis.