Design and operation of petroleum field facilities require more and more advanced flow calculations to optimize investments and maximize production and reserves. A number of integrated software platforms provide useful architectures to model multiphase flow from well-bore to topside facilities. Even through the use of such modeling technologies, the complexity of oil & gas production systems remains a strong obstacle to manually screen the system settings for optimal operation and design: the use of an optimizer is well suited in that case. The paper discusses the integration of Sequential Quadratic Programming optimization module developed by ProSim SA (thereafter called OPTI) into the CAPE-OPEN compliant simulation environment INDISSTM developed by RSI. OPTI resolves non-linear optimization problems. For its integration into INDISSTM, OPTI has been wrapped as a CAPE-OPEN unit operation (called CO-OPTI) that can be linked with other unit operations on the flowsheet thanks to CAPE-OPEN (CO) standard interfaces. An example based on a deepwater production system illustrates the benefit of this development.