The objective of this work is to quantify the controllability of a complex heat-integrated reactor. Similarly to the concept of ultimate gain in the control literature, the ultimate effectiveness is defined for a complex feed-effluent heat exchanger (FEHE) scheme. This parameter indicates the amount of heat that can be recovered (via FEHE) before the overall open-loop system becomes unstable. First, a systematic approach is proposed to model the complex heat-integrated reactors. A simple measure, the overall effectiveness, can be derived directly from the flowsheet. Given the reactor model, the controllability of a particular flowsheet can be evaluated on the basis of the stability margin of design. An interpretation based on the heat-generation and heat-removal curves is also given. With the controllability measure, implications for design are also explored. Because the loss of controllability comes from the positive feedback loop, several design parameters are studied, and design heuristics are proposed to improve the controllability of heat-integration schemes. Two examples, a simple two-FEHE example and an HDA example, are used to assess the controllability of different designs. The results show that, contrary to one's intuition, some of the complex heat-integrated reactor design alternatives (e.g., alternatives 6 and 7 of the HDA example) are indeed more controllable than the simpler heat-integration schemes (e.g., alternative 1). The increased number of FEHEs allows for a greater number of candidate manipulated inputs and thus provides opportunities for multivariable control. Contrary to one's intuition, the multivariable controlled FEHE/reactor system gives a steeper slope and a lower peak for the closed-loop load-transfer function. This results in an improved disturbance rejection capability.