Computing the worst-case execution time (WCET) of real-time tasks is mandatory for the correct functioning of real-time systems. In component-based development (CBD) of real-time embedded systems, a task is typically constructed by composing appropriate software components together. Finding the WCET of a component-constructed task reduces to finding the WCETs of its constituent components and composing them in an appropriate fashion. In this paper we describe a method based on constraint-logic programming (CLP) to produce WCET estimates of real-time tasks where the components' WCETs are expressed parametrically. The experimental results show that the derived WCETs using our approach benefit from an increased tightness while the practical (time) complexity of the approach is manageable.