ReactiveX observable that uses another observable
I had to use flatMap to achieve what I wanted:

public get(url: string): Observable&lt;Response&gt; {
  return this
    .login()
    .flatMap(token_id =&gt; this._http.get(url));  // token_id can be used here
}

public login(): Observable&lt;string&gt; {
  const authBody =
      {
          "client_id": "...",
          "username": "...",
          "password": "...",
          // ...
      };
  const headers = new Headers();
  headers.append("Content-Type", "application/json");

  return this._http
    .post('AUTH_URL', authBody)
    .map(response =&gt; response.json().token_id);
}