Combine first 9 ingredients in a large saucepan; bring to a boil. Reduce heat; simmer 25 minutes or until vegetables are tender. Discard lemongrass. Cool slightly. Pour 2 cups potato mixture in a blender; process until blended. Pour into a bowl. Repeat procedure with 2 cups potato mixture. Return pureed mixture to pan. Stir in chicken and remaining ingredients; cook over medium heat 5 minutes or until thoroughly heated.