Reliable mathematical models for an industrial steam-gas reformer and a steam-boiler are simulated and used to study the dynamic behavior of the two coupled processes. Simulation results of the identified system are in good agreement with the nonlinear process operation. Three control loops are considered for multivariable control system design. These are Hydrogen product temperature and quality, and boiler water level. A new multivariable control structure is obtained, which manipulates steam-to-carbon (S/C) ratio for the control of coil outlet temperature (COT), the fuel gas rate to control hydrogen product quality (conversion) and the boiler feed water to control drum level. The selected structure is then tuned using the Biggest-Log modulus-Tuning (BLT) method. Results show a very satisfactory response of the temperature and quality loops with the BLT based controllers. It is also found that the boiler water level loop is partially decoupled from the other two loops and hence does not need to be detuned according to BLT criterion. The new multivariable control structure is compared with the conventional control utilising fuel gas rate to control COT. Closed-loop simulation results show a better performance for the multivariable structure under continuous operation.