Bring water to a boil in a saucepan. Add lemon grass, galangal, and kaffir leaves. Cook for 1 minute. Add fish sauce and cook for 1 minute. Add chile paste and reduce heat to medium. Add tomatoes and cook 2 to 3 minutes. Add mushrooms and cook for 1 minute. Increase heat to high and add chicken. Cook, stirring occasionally, until chicken is cooked through, about 5 minutes. Add lime juice and stir.
