Many software systems fail as a direct consequence of errors in requirements analysis. Establishing formal metrics early in the design process, using attributes like critical parameters, enables designers to properly assess software success. While critical parameters alone do not have the potential to drive design, establishing requirements tied to critical parameters helps designers capture design objectives. For the design of interactive systems, the use of scenario-based approaches offers natural user centricity and facilitates knowledge reuse through the generation of claims. Unfortunately, the requirements-analysis phase of scenario-based design does not offer sufficient built-in and explicit techniques needed for capturing the critical-parameter requirements of a system. Because success depends heavily on user involvement and proper requirements, there is a crucial need for a requirements-analysis technique that bridges the gap between scenarios and critical parameters. Better establishing requirements will benefit design. By adapting task-modeling techniques to support critical parameters within the requirements-analysis phase of scenario-based design, we are able to provide designers with a systematic technique for capturing requirements in a reusable form that enables and encourages knowledge transfer early in the development process. The research work presented concentrates on the domain of notification systems, as previous research efforts led to the identification of three critical parameters. Contributions of this work include establishment of a structured process for capturing critical-parameter requirements within a user-centric design framework and introduction of knowledge reuse at the requirements phase. On one hand, adapting task models to capture requirements bridges the gap between scenarios and critical parameters, which benefits design from user involvement and accurate requirements. On the other hand, using task models as a reusable component leverages requirements reuse which benefits design by increasing quality while reducing development costs and time-to-market.