This paper describes a scalable approach to the enabling of legacy scientific applications on computing Grids using a service- oriented architecture. In the context of this paper Grid enabling means turning an existing application, installed on a Grid resource, into a service and generating the application-specific user interfaces to use that application through a web portal. Scalability is achieved by providing a common abstraction for a category of applications and providing a "generic" application service to wrap those applications as services. The focus of this paper?s approach is on Grid-enabling "command-oriented" scientific applications. The novel aspect of the approach is that the entire process - from turning an application into a service to the user-interface generation for that application - is done automatically, without requiring coding or Grid-system downtime. Portlet technology is used to dynamically generate application-specific interfaces. Further, the approach makes it possible to customize the applications for different user groups by way of simplifying, restricting or composing the functionalities of applications. The approach is useful for building Grid portals on which a large number of applications need to be dynamically enabled.